1. Field of the Invention
The invention relates to a particle-optical apparatus which comprises a particle source for producing a primary beam of electrically charged particles which propagate along an optical axis of the apparatus, the beam being controlled to scan a specimen to be examined. The apparatus further comprises a focusing device for forming a beam focus in the vicinity of the area in which the specimen is to be arranged, which focusing device is constructed as a combination of a gap lens and a monopole lens whose lens fields have been shifted relative to one another along the optical axis, and a detection device for detecting electrically charged particles originating from the specimen, at least a part of said detection device being situated within the focusing device.
2. Description of the Related Art
An apparatus of this kind is known from Japanese Patent Application No. 2-24336 filed, on Feb. 2, 1990, and published Oct. 14 1991 under No. 3-230464.
The apparatus described in the cited Japanese Patent Application is a scanning electron microscope (SEM). Microscopes of this kind are used to form electron optical images of a specimen by scanning a focused electron beam across the specimen. The known apparatus comprises a beam focusing device formed by a combination of a conventional magnetic gap lens and a monopole lens.
The lens field in the conventional magnetic gap lens is generated by a gap in the iron circuit of the lens; the lens field in the monopole lens is generated between an end of the iron circuit of the lens and its vicinity, notably the specimen to be examined in the apparatus. The lens field of the magnetic gap lens is situated above the lens field of the monopole lens in this known apparatus.
Lenses of the monopole type have an iron circuit which at the end facing the specimen is shaped as a funnel which is rotationally symmetrically about the optical axis. A lens coil provides excitation of this lens in such a manner that the magnetic field emanates from the pole material at the area of the narrowest part of the funnel. The required lens effect is thus obtained at that area, offering the known advantage of this type of lens that the lens effect occurs in an environment in which pole material of the electron lens is absent. This lens can therefore be used to make electron optical images of a specimen by scanning a focused electron beam formed by the lens across the specimen. This type of lens is thus customarily used in scanning electron microscopes. Therein, the specimen to be examined is rigidly arranged relative to the lens pole, for example at a distance of 1 mm from the end of the funnel-shaped lens pole.
The accelerating voltage of the electron beam in a SEM is chosen in dependence on the nature of the specimen to be examined. This accelerating voltage must have a comparatively low value (of the order of magnitude of 1 kV) in order to counteract charging of the specimen by the primary electron beam as much as possible. This could occur, for example during a study of electrically insulating layers in integrated electronic circuits or in the case of given biological specimens. Other specimens, however, require a higher accelerating voltage, for example of the order of magnitude of 30 kV. For these high accelerating voltages the resolution (of the order of magnitude of 1 nm) of a gap lens suffices; however, it is a known property of a gap lens that the chromatic aberration of this type of lens unacceptably degrades the resolution in the case of said lower accelerating voltages. For such low accelerating voltages a monopole lens is required which offers a suitable resolution (again of the order of magnitude of 1 nm).
In a SEM a specimen is usually observed by studying secondary electrons of low energy (of the order of magnitude of 5-50 eV) released from the specimen by the primary electron beam. These low-energetic electrons are accelerated in the direction of a detector. When use is made of a monopole lens which produces a magnetic field which extends as far as the specimen, these electrons perform a spiral-like motion in the vicinity of the specimen, around a magnetic field line which is tangent to the specimen at the area where the electron leaves the specimen. However, if a detector is arranged in the monopole lens, it will be reached only by those secondary electrons which have propagated along a magnetic field line starting from a point within the monopole lens; all other secondary electrons are not "seen" by the detector. Because of this effect, the field of vision of an electron microscope comprising a monopole lens is limited to approximately 0.1.times.0.1 mm.sup.2 in the case of a free working distance of 1 mm. Because of this small field of vision of the monopole lens, the use of the gap lens (having a field of vision of a few mm.sup.2) is important in the case of low accelerating voltages: it is then possible to select a region of the specimen to be examined by means of a large field of vision, after which this region can be studied in detail by means of the (high-resolution) monopole lens. Combining the monopole lens and the conventional gap lens so as to form one focusing device thus enables selection of the required accelerating voltage for a given specimen. The excitation of the gap lens can be deactivated and the monopole lens can be excited without it being necessary for the user of the microscope to make any changes in the apparatus (notably without it being necessary to change the vertical position of the specimen).
A further advantage of this focusing device consists in that it offers space to accommodate a detector for secondary electrons. Secondary electrons emanating from the specimen at the lens side can then be observed by this built-in detector.
In the known SEM the detector for secondary electrons is mounted above the monopole field as well as above the lens field of the gap lens. This arrangement of the detector has a drawback which will be described hereinafter.
Secondary electrons released from the specimen by the primary beam exhibit a directional distribution which has a maximum for an angle of 45.degree. relative to the specimen surface, whereas only a very small fraction of the secondary electrons emanates in the direction perpendicular to this surface. (For a more detailed description of the directional dependency of the secondary electrons reference is made to a book by L. Reimer "Scanning Electron Microscopy" (notably page 186), Springer Verlag, 1985, ISBN 3-540-13530-8.)
The drawback of the known configuration of lenses and detector arises in that the electrons must pass the lens field of the gap lens in order to reach the detector. If only this lens is in the excited condition, i.e. when the electron beam is focused on the specimen by this lens, the lens field of this lens will be comparatively strong and the magnetic field at the area of the specimen will be many tens of times weaker or may even be considered to be substantially equal to zero for all practical purposes. When external electrons (i.e. electrons from the specimen in which the magnetic field strength is very low) are accelerated into the lens field, only those electrons which propagate substantially parallel to the optical axis can actually traverse the lens field. All other electrons are forced to return in the lens field and, therefore, will not reach the detector. This phenomenon is known as the magnetic bottle effect. (For a more detailed description of this phenomenon, reference is made to a publication "Magnetic Through-the-lens Detection in Electron Microscopy, part I" by P. Kruit in "Advances in Optical and Electron Microscopy", Vol. 12, ISBN 0-12-029912-7, 1991, Academic Pres Ltd, notably chapter I "Introduction" and the associated FIG. 1.) In the case of secondary electrons, therefore, only electrons which have emanated from the specimen surface in the perpendicular direction can traverse the lens field so as to reach the detector. As has already been described, this applies to only a small fraction of the total number of secondary electrons, so that the detection efficiency will be very low because of this phenomenon.